I think it makes a lot of sense right now for the 3rd parties to go exclusive without putting too much of a charge on Sony.
The thing again is gamepass. Consider the impact that makes to sales of games on gamepass platforms .... people are talking up and down about how gamepass saves money - but in only does that if you don't also buy full price games. They'll play game pass games and wait for stuff to come to that service.
If a 3rd party AAA developer wants to make $60 games on gamepass platforms, they have to compete with a whole load of cheap shovelware, XGS GaaS content and the one or two old, big budget games MS signs to the service.
It's not a level playing field to compete for $$$. It's like a major movie competing with the profusion of Netflix originals. To make that money back the movie needs a big theatrical release and the ticket sales at the box office.
On Sony platforms right now, it is a level playing field (at least Sony haven't announced any changes to that yet). So I can see why third parties might feel they should go exclusive to PS for a period - they can drive full price sales and get the bump that comes with the exclusive tag, then release on Xbox down the road and perhaps jump onto gamepass to hoover up some cash from that.
Until we see how sales of HW and SW shake out post launch it's difficult to be sure, but I think 3rd party devs who don't want to be part of gamepass - that would be all the big producers - are probably wary of the competitive landscape on Xbox.